A broad, relatively flat region of the ocean that lies at least 4.5km below sea level.

Apparent polar-wander path

A path on the globe along which a magnetic pole appears to have wandered over time; in fact, the continents drift, while the magnetic pole stays fairly fixed.

Bathymetry

Variation in depth (underwater topography).

Continental drift

The idea that continents have moved and are still moving slowly across the Earth's surface.

Dipole

A magnetic field with a north and south pole, like that of a bar magnet.

Dynamo

A power plant generator in which water or wind power spins an electrical conductor around a permanent magnet.

Fracture zone

A narrow band of vertical fractures in the ocean floor; fracture zones lie roughly at right-angles to a mid-ocean ridge, and the actively slipping part of a fracture zone is a transform fault.

Heat flow

The rate at which heat rises from the Earth's interior up to the surface.

Magnetic anomaly

The difference between the expected strength of the Earth's magnetic field at a certain location and the actual measured strength of the field at that location.

Magnetic declination

The angle between the direction a compass needle points at a given location and the direction of true north.

Magnetic field

The region affected by the force emanating from a magnet.

Magnetic force

The push or pull exerted by a magnet.

Magnetic inclination

The angle between a magnetic needle free to pivot on a horizontal axis and a horizontal plane parallel to the Earth's surface.

Magnetic reversal-chronology

The history of magnetic reversals through geologic time.

Magnetic reversals

The change of the Earth's magnetic polarity; when a reversal occurs, the field flips from normal to reversed polarity, or vice versa.

Magnetite

An iron-rich mineral that acts like a permanent magnet.

Magnetisation

The degree to which a material can exert a magnetic force.

Mid-ocean ridge

A 2km high submarine mountain belt that forms along a divergent oceanic plate boundary.

Normal and reversed polarity

Polarity in which the palaeomagnetic dipole has the same orientation as it is today / points north.

Palaeomagnetism

The record of ancient magnetism preserved in rock.

Palaeopole

The supposed position of the Earth's magnetic pole in the past, with respect to a particular continent.

Pangaea

A supercontinent that assembled at the end of the Palaeozoic era.

Plate

One of about twenty distinct pieces of the relatively rigid lithosphere.

Plate tectonics

The theory that the outer layer of the Earth (the lithosphere) consists of separate plates that move with respect to one another.

Polar wander

The phenomenon of the progressive changing through time of the position of the Earth's magnetic poles relative to a location on a continent; significant polar wander probably doesn't occur - in fact, poles seem to remain fairly fixed, while continents move.

Polarity

The orientation of a magnetic dipole

Polarity chron

The time interval between polarity reversals of Earth's magnetic field.

Sea-floor spreading

The gradual widening of an ocean basin as new oceanic crust forms at a mid-ocean ridge axis and then moves away from the axis.

Seamount

An isolated submarine mountain.

Spreading rate

The rate at which sea floor moves away from a mid-ocean ridge axis, as measured with respect to the sea floor on the opposite side of the axis.

Subduction

The process by which one oceanic plate bends and sinks down into the asthenosphere beneath another plate.

Trench

A deep elongate trough bordering a volcanic arc; a trench defines the trace of a convergent plate boundary.

Volcanic arc

A curving chain of active volcanoes formed adjacent to a convergent plate boundary.